ABSTRACT Dendritic and spine plasticity plays key roles in brain development, function, behavior, and disease. Indeed, spine and dendrite pathology is a common feature of many neuropsychiatric disorders (NPDs), including autism spectrum disorder (ASD), schizophrenia (SZ), and bipolar disorder (BPD). Rho-like small GTPases, including Rac1, are a family of regulatory proteins with central roles in dendrite and spine plasticity. Their extensive implication in NPDs suggests that these pathways can serve as therapeutic targets in NPDs. The activity of small GTPases is enhanced by guanine-nucleotide-exchange factors (GEFs), among which the Rac1- GEF kalirin is highly enriched in spines, and is perhaps the best-characterized GEF in the brain. Kalirin is a central regulator of dendrite arborization, spine plasticity, glutamatergic transmission, neuronal connectivity, and cognitive behavior. While small-molecule pharmacological modulators have been invaluable tools for studying the biological functions of kinases, receptors, or ion channels, no such tools exist for Rho-GEFs, including kalirin. Kalirin is an optimal drug target for several reasons: its expression is largely restricted to the CNS, it is highly enriched in spines, it is a signaling hub in a synaptic network including many NPD risk factors, its enzymatic activity can be modulated, and the 3D structure of its GEF domain has been determined. Here we outline a novel and innovative hit validation cascade that will allow us to develop small-molecule tools to investigate a previously unapproachable target relevant to NPDs. The brain-specific expression of kalirin and its highly compartmentalized subcellular localization at synapses suggests that regulation of Rac1 signaling, through pharmacological interventions targeting kalirin, may allow neuron- and synapse-specific effects. This is key to developing tools that produce cell type-specific and context-dependent Rac1 modulation. Using a combination of high-throughput screening (HTS) and in silico screening against the target proteins kalirin/Rac1, we produced a hit list of potential regulators of kalirin activity suitable for follow-up analysis in well-characterized and optimized assays. We hypothesize that small-molecule compounds isolated in HTS and in silico screens modulate kalirin's GEF and biological activity in rodent and human iPSC-derived neuron models, and reverse neuroarchitectural abnormalities in models of NPDs. We will test this hypothesis in the following aims: 1) Hit validation and in vitro characterization, selection, and prioritization. 2) Prioritization of mouse and human neuronal model systems for testing validated hits. 3) Characterization of validated hit compounds in mouse and iPSC models.